(a) Field of the Invention
The present invention relates to a current sample-and-hold circuit. More specifically, the present invention relates to a current sample-and-hold circuit for demultiplexing data current in a current driven display device.
(b) Description of the Related Art
FIG. 1 shows an AMOLED (active matrix organic light emitting diode) display device as an example of a current driven display device which requires current demultiplexing. Exemplary embodiments of the present invention can be applied to the AMOLED display device of FIG. 1, as well as other suitable current driven display devices.
The current driven display device of FIG. 1 includes a data driver 10 for supplying the data current, a demultiplexer 20 for demultiplexing the data current by a ratio of 1:N, and scan drivers 31 and 32 for sequentially selecting select lines of a pixel 40 on a predetermined row.
The pixel 40 belonging to the line selected by the scan drivers 31 and 32 receives a predetermined data current, and represents a color which corresponds to the data current. In the current driven display device of FIG. 1, a current demultiplexer 20 for reducing the number of the data drivers 10 is used. That is, the current provided by the data driver 10 is demultiplexed by 1:N by the demultiplexer 20 to thus supply data to the pixels which correspond to N data lines 50. The number of required data drivers is reduced by using the demultiplexer 20, thereby reducing purchase costs.
FIG. 2 shows an analog switch used for a conventional demultiplexer.
In the 1:2 demultiplexer of FIG. 2, the data current is output to two data lines as switches S1 and S2 are alternately turned on and off. In order to realize high resolution in a current driven panel, a long time is required to write the data on the pixel 40. However, since the data is to be written on the pixel each time the switches are switched, the data writing time needs to be reduced when reducing the number of the data drivers.
Therefore, it is difficult to realize high resolution using a conventional demultiplexer. This difficulty is addressed by configuring the demultiplexer using a sample-and-hold circuit.
FIG. 3 shows a conventional sample-and-hold circuit 120.
A pixel circuit of the conventional AMOLED has an input terminal and an output terminal in the form of a current source. Therefore, the output terminal of the sample-and-hold circuit 120 should be in the form of a current sink since the input terminal of a pixel 140 is coupled in the current source format to an external device.
Since the conventional pixel circuit stores current programmed data and writes the same on the pixels, the pixel circuit performs a similar function of the sample-and-hold operation.
In the case of using the conventional pixel circuit as a sample-and-hold circuit, both of the input terminal and the output terminal of the sample-and-hold circuit 120 are implemented in the source format or the sink format.
Hence, the conventional sample-and-hold circuit to be used with the pixel 140 having the input terminal in the source format has the output terminal in the current sink format, and in this instance, the input terminal of the sample-and-hold circuit is also in the current sink format and coupled to a data driver IC 110 having an output terminal in the current source format.
Accordingly, as shown in FIG. 3, in the case of using the conventional sample-and-hold circuit 120, the output terminal of the data driver IC 110 has a current source format. However, the driver ICs having the output terminals in the current sink format have less production costs compared to the driver ICs having the output terminals in the current source format. Also, the driver ICs having the output terminals in the current sink format have further universal purposes since they can be directly coupled to the pixels configured in the current source format. In addition, the driver ICs having the output terminals in the current sink format expend less power, occupy less areas, and generate substantially uniform outputs.
However, in the case of using the sample-and-hold circuit 120 in the current driven display device which has the same operational conditions as those of the AMOLED, the advantages of the driver ICs having the output terminals in the current sink format are lost.